From what’s publicly available: OVH, Equinix, NetActuate, CacheNet (ref). Fly uses AWS (S3, RDS etc), but for control-plane, iiuc.
Kurt says, pound-for-pound dedicated-cpu on Fly is faster: Heroku equivalents for PL Dyno - #2 by kurt
With shared-cpu, of course, there’s only a limited timeslice available to a VM, which is ~1/15th for Fly’s lowest tier, share-cpu-1x: Heroku Free Tier Migration - #6 by ignoramous